New applications requiring printed conductive materials are continuously arising in the electronics industry. Some of the newly developing applications are printed antennas for radio frequency identification (“RFID”) tags, printed transistors and solar cells. Such applications, along with much of the electronics market, are driven by cost and speed of assembly. Consequently, conductive materials are required that are capable of high throughput. High throughput is epitomized by high speed printing techniques such as flexography and rotogravure which are increasingly utilized instead of the slower screen printing process. For example, production speeds of up to about 400 meters per minute may be achieved through the high speed printing techniques, as opposed to speeds in the range of about 60 meters per minute via rotary screen printing. As such high speed techniques are becoming increasingly common in the packaging, consumer and publication industries, conductive materials must be adapted to have the proper rheological properties to be utilized at such high speeds.
Commercially available electrically conductive materials have traditionally been in the form of polymer thick films (“PTFs”). These PTFs are generally high viscosity materials that are applied to a substrate by screen printing to form electronic circuits in items such as membrane keyboards, printed circuit boards and heating elements. Most PTFs consist of a resin either in solvent or dispersed/dissolved in water along with conductive filler, with silver being the preferred filler for most high conductivity applications. After application the water or solvent is evaporated through drying or curing and the conductive filler forms interparticle contacts to create a conductive network. In addition to the solvent and waterbased systems, ultraviolet (“UV”) curable PTFs are commercially available. Such UV curable PTFs generate conductivity by curing the film with UV or electron beam (“EB”) radiation after application. UV curable materials generally cure at ambient temperatures and at a faster rate than thermally cured materials. UV curable materials have a further advantage in that the release of environmentally harmful substances is minimized during curing.
Accordingly, it would be advantageous to provide a conductive material that is capable of UV curing with rheological properties that are amenable to high speed printing techniques, such as flexography and rotogravure.